The invention relates to an installation for recovering energy from solid fossil fuels, more particularly fuels high in inerts and particularly bituminous coal, the said installation consisting of at least one unit in which the solid fuels are converted into gas, a gas-turbine and a steam-turbine being provided to recover the energy from the gases, and the said gases being freed from dust and desulphurized in the unit before the gas-turbine.
The advantage of such units is that, with a suitable combination of the gas-turbine and steam-turbine processes, they provide higher thermal efficiency than a conventional arrangement of individual processes. The dust-removal serves to clean the gases to such an extent that although the coal used is high in inerts, the said gases may be fed to the gas-turbine. The desulphurization is carried out at a high pressure level and therefore has advantages over known unpressurized flue-gas desulphurization, in that the desulphurizing units are smaller and have lower losses; in the case of adsorptive desulphurization, the adsorption losses are reduced by the increased pressure. For this reason, installations of this kind cause very little pollution.
So-called solid-bed pressure gasification is already known. In installations of this kind, coal, mostly high in the inerts is gasified, i.e. partly burned, with some of the combustion air available, and with steam, under high pressure. In known installations of this kind, the gasification pressure is about 20 bars. The gasifications is carried out in a reactor which produces lean gas at a temperature of between 500.degree. and 600.degree. C., which is cooled and then passes to dust-removing and desulphurizing units. The cleaned lean gas is burned as fuel gas in a boiler under pressure, the flue-gases from which are used to operate the gas-turbine, while the steam produced in the boiler drives the steam-turbine.
The pressure-gasification of coal, however, results in a series of losses. Some of these are due to the relatively large amount of unburned fuel in the ash removed from the reactor, which has hitherto amounted to more than 10%. Further losses arise from so-called jacket steam, i.e. evaporation of the cooling water fed to electrical pressure-gasification unit. Other losses are due to the evaporation of water used in so-called quenching. Finally, still further losses are due to the fact that an appreciable amount of the water used in washing the gases is never removed, but remains in the fuel gas in the form of spray and must therefore be evaporated in the firing of the boiler.
Other disadvantages associated with the pressure-gasification of coal arise from operating difficulties. During the cooling of the lean gas, tar is condensed and this deposited on the dust which is also present in the lean gas. This produces a mixture of dust and tar which soon leads to incrustation and blocking of the various circuits in which it occurs, and from which it therefore has to be removed. If substantial heat losses are to be avoided, the mixture of dust and tar must be separated from the washing medium in a tar-separator or the like unit, and must be returned to the gas-producer. This returning of the tar arises problems, since the dust-containing tar can be pumped only to a limited extent and this always causes operational problems.
Pressure gasification units for coal hitherto used are also relatively difficult to control. More particularly, the gas-discharge temperature, calorific value, dust content, and tar content are not very constant, which is attributable mainly to the intermittent supply of coal to the gas-producer. Furthermore, a rapid load increase is impossible if an adequately high calorific value of the fuel gases is required at the same time. These problems make an adequately sensitive output control impossible.
Finally, it should be pointed out that, because of their low calorific value, the fuel gases also cause problems during combustion in the boiler, and there are always occasions when combustion has to be assisted with additional fuels, more particularly fuel oil.
Also known is an installation of the type described at the beginning hereof in which the fuel is burned and desulphurized in a fluidized bed. To this end, the ground fuel and desulphurizer are fed to the fluidized bed and the fuel is burned under pressure, in suspension. The transfer of heat to the steam process is carried out within the fluidized bed, whereby the combustion temperature is restricted to about 900.degree. C. The flue-gas emerging from the fluidizing chamber, and containing large amounts of ash, unburned fuel, and partly charged desulphurized, must then be fed to a gas-turbine. So far there is no known way of producing a gas clean enough for a machine from these flue gases.
If the approach used is to burn the ground fuel in a fluidizing chamber, this has numerous disadvantages, since considerable amounts of unburned fuel in the ash must also be expected. One particularly difficult problem is that the unburned fuel is discharged from the fluidizing chamber with the flow of flue-gas and therefore inevitably reaches the subsequent dust-removing unit. Separating the ash produced by the fuel, and the partly charged desulphurizer, from the flow of flue-gas presents considerable difficulties. Another problem is separating the partly-charged desulphurizer from the ash in order to condition the desulphurizer. In addition to this are wear problems at the heating surfaces which have to be arranged in the fluidized bed. This wear is caused mainly by the erosion which inevitably occurs in a fluidized bed. Accurate control of fluidized-bed temperatures also causes problems, since if these temperatures become too high, the ash softens and bakes onto the fluidized bed and heating surfaces, whereas if the fluidized-bed temperature is too low, there is a reduction in combustion, i.e. an increase in fuel losses. Partial carbonization also occurs, which in turn leads to tar condensation and to baking onto the fluidized bed and heating surfaces.